main rotor aerodynamics

7/30/2019 Main Rotor Aerodynamics

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CONTENTS

PART I

1. BASIC AERODYNAMICS

PART II

2. METEOROLOGY

PART III

3. NAVIGATION


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PART I

BASIC AERODYNAMICS

1. DEFINITIONS

2. FLAPPING TO EQUALITY

3. HOOKS JOINT EFFECT

4. DISSYMMETRY OF LIFT

5. TAIL ROTOR DRIFT

6. TAIL ROTOR ROLL

7. LOSS OF TAIL ROTOR EFFECTIVENESS

8. GROUND CUSHION

9. RECIRCULATION

10. FLARE AND ITS EFFECTS

11. DANGEROUS CURVES

12. BLADE SAILING

13. LIMITS OF RPM

14. ADVANCE ANGLE

15. POWER REQUIREMENT


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DEFINITIONS

1. Maximum All Up Weight. The maximum weight

at which the aircraft is permitted to fly within normal

design restrictions. Found in the LIMITATIONS

section of the Aircrew Manual.

2. All Up Weight (auw). The actual weight of the

aircraft at any given time. AUW must not be greater than

MAUW.

3. Basic Weight. The actual weight of the

aircraft with its basic equipment including oil, hydraulic

fluid, fire extinguishers, first aid pack, etc, but excluding

specific role equipment, fuel and crew. Found in thedocuments of the aircraft.

4. Variable Load. Items, which may vary from

sortie to sortie but are not expendable on flight, e.g.: crew

and role equipment (winch).

5. Expendable Load. Items such as fuel, oil,

weapons and other cargo/stores which may be

airdropped, including parachutists.


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6. Payload. The total load of passengers/cargo

actually carried in the aircraft.

7. Operating Weight (Role operating weights

excluding fuel). The sum of basic weight and variable

load which when subtracted from MAUW provides your

LIFTING CAPACITY. Lifting Capacity determines the

compromise between expendable load and payload.


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FLAPPING TO EQUALITY

1. Definition. Moving the cyc stick does not alter

the total rotor thrust but simply changes the disc att. This

is achieved by the blades flapping to equality when the

cyc pitch is applied the flapping to blades can be defined

as angular movement of blades above and below the plan

of hub.

2. Suppose a hel is hovering in ideal wind conditions

and one of its blade is having an angle of attk 6.

3. Now we mov the cyc stick to fwd posn. This mov

of cyc will decrease the blade pitch and this decrease in


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AOA of blade is due to mechanical linkages or due to

con orbit. Here we consider that RAF remain unchanged

this reduction in pitch will reduce both the blades AOA

and rotor thrust, and when the rotor thrust is reduce

therefore lift of the blade will reduce so the blade will not

be able to maint its original horizontal flight and will

definitely begin to flap down.

4. Now when the blade is falling down there will be

some flow which is coming up we call it up flow. Now

this up flow will be resisting the induced flow and

causing its reduction till its AOA is reached to 6, and


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blade thrust will return to its original value and the blade

will continue to follow the new path required to keep the

AOA constant.

Thus cyc pitch will alter the plane in which theblade is rotating but AOA remains unchanged.

5. The reverse takes place when a blade experiences

an increase in AOA when the cyc stick is mov aft.


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HOOKS JOINT EFFECT

1. Definition. Hooks joint effect is defined as the

mov of blade to reposition itself relative to the other

blades when the cyclic stick is applied.

2. Explanation

a. GM, today we will be studying a very

important cause of dragging of main

rotor blade that is Hook Joint Effect.

Before we see what is Hook Joint

Effect. Let us see what is dragging.

Dragging is the ability or freedom

given to each blade to lead or lagindependently of the other blade. The

causes of dragging are periodic drag

changes, changing posn of CG

relative to the hub and Hook Joint

Effect.


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b. Now let us consider a four bladed

rotor system is rotating in POR. If the

cyclic stick is maint at neutral posn,

the blades will maint track as shown

in fig if viewed from above.

B

Figure 36 Hooks Joint Effect

AA

B

C C

DD

Shaft

AxisShaft

Axis

Tip Path Plane

Original Tip Path

Plane

New Tip Path Pla


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3. Now if the cyc is moved fwd the disc will tilt in

the fwd dir and if still viewed from above. Now the blade

A would have increased its radius from the center of the

hub where as the blade C would have decreased it radius

relative to the center of hub.

4. Consider the law of conservation of angular

momentum that is M = m Vr where m is the mass of the

blade V is rotational velocity and r is the radius of the

blade. Now when ever the blade is having closer dist i.e.

(radius) to maint the momentum constant velocity V has

to be increased.

5. As the blade C increases its Vr and blade Adecreases its Vr the blade B, D would try to gain their

original path and while doing so blade B would try to

speed up as the blade C has an increased Vr and blade D

would try to slow down as blade a has low velocity.

It can be seen, that the blade which is moving with

high V, has a tendency to lead and the blade moving with

less V has a tendency to lag in their planes of rotation.


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DISSYMMETRY OF LIFT

SYMMETRY OF LIFT

1. Definition. A structure that allows an object

being divided by a point or line or plane into two or more

parts exactly similar in size and shape and in position

relatively to the divided point.

2. Still Air Condition. Let us consider for a

moment that the helicopter is hovering in still air

condition, the rotor thrust produced by each blade will be

uniform. The speed of RAF over each blade will be equal

to the speed of rotation of each blade irrespective of its

position throughout the blades 360 degree of travel.Therefore it can be said that the RAF for each blade is

Vr, that is rotational velocity of the blade. In this

condition where all the factors affecting the lift are

constant, the blades will experience same lift throughout

their 360 degree of travel.


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DISSYMMETRY OF LIFT

3. Suppose the condition has changed and the

helicopter is now facing into the wind, with a velocity of

Vw, during hover. Half of the time the blade will be

moving into the wind and for the remainder time it will

be moving along the wind. The disc can therefore be

divided into two halves, one half being the advancing

side, and the other retreating side.

4. When the rotor blade starts moving from point A

towards advancing side, as it moves the RAF will start

Vr

V

r

V

r

V

rVw = Wind Velocity

Vw

Vw RAF

Wind


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increasing and a component of wind velocity will also

start supplementing the rotational velocity Vr. The RAF

will attain its maximum value of (Vr+VW) when the

blade reaches point B. As the blade continues to rotate

the value of Vw will start decreasing. Once the blade

reaches point C the value of Vw will again be Vr.

5. From point C as the blade moves forward it enters

the retreating side where the wind Vw is acting along

rotational velocity Vr, and partially canceling the effect

of Vr. As the blade is advancing on the retreating side the

value of RAF will keep on reducing by an amount Vw

and will reach its lowest value of (Vr-VW)once it

reaches 90 degrees on the retreating side that is at

point D.

6. If no change takes place in blades attitude than the

advancing blade at point B will produce lift L = CL 1/2

(Vr+Vw)2 S and the retreating blade at point D will

produce L = CL (Vr-Vw)2 S. The value of lift on

advancing side being more than the retreating side. This


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condition where one side of the disc produces more lift

than the other is known as dissymmetry of lift.


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TAIL ROTOR DRIFT

1. If a fuselage is being turned by a couple yy about a

point x the rotation will stop if a couple zz of equal value

pulls it in the opposite direction.

2. The rotation will also stop if a single force zz was

used to produce a moment equal to the couple yy but

there would now be a side force on the pivot point x.

3. The tail rotor of a helicopter produces a moment to

overcome the couple arising from torque reaction which

in turn causes a side pull on the pivot point or axis of

rotation of the main rotor. This sideward force produces a

movement known as tail rotor drift and unless corrected,it would result in the helicopter moving sideways over

the ground.

YY

X XX

Z Y

ZZZ Z

YY


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4. Correction for Tail Rotor Drift. Main rotor

thrust is offset to produce a side thrust to correct for tail

rotor drift. This is achieved either automatically through

design or by the pilot tilting the disc in required direction.

5. Implication. Implication of TR drift can be

well understood if we carefully monitor the flying

techniques of Schweizer helicopter during under

mentioned phases of its operation :-

a. During normal hover the cyclic stick has to

be placed slightly left, it is because of the

drift corrective force.b. Once carrying out hovering auto we have to

ease the cyclic towards right to avoid hel

drift towards left, it is because the engine

torque during auto is no more there and

correspondingly the drift corrective force

will finish.


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TAIL ROTOR ROLL

1. As the tail rotor roll is by-product of tail rotor drift,

the drift corrective force acts at the main rotor hub center,

in this situation tail rotor thrust is in the opposite

direction. If the tail rotor is mounted below the horizontal

level of the main rotor hub, a couple is formed between

tail rotor thrust and tail rotor drift correcting force.

2. This rolling couple causes the helicopter to hover

one wheel low.

3. Compensation. The tail rotor roll can be

compensated by mounting it in level with main rotor hub.

This is achieved by :-a. Cranking the fuselage to the level of main

rotor hub.

b. Fitting tail rotor to a pylon to raise it to the

level of main rotor hub.


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Rolling Couple

Tail Rotor

Thrust

Total

RotorThrust

Tail Rotor Drift

Correcting Force

Weight


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LOSS OF TAIL ROTOR EFFECTIVENESS

1. Unanticipated right yaw or loss of tail rotoreffectiveness (LTE) has been determined to be a

contributory factor in a number of accidents. In most

cases inappropriate or late corrective actions may have

resulted in the development of uncontrollable yaw. These

mishaps have occurred at low altitude, low airspeed flight

region while manoeuvring, on final approach to a

landing, or during nap of the earth tactical terrain flying.

Severity of this problem depends upon the characteristics

of tail rotor or simply the resultant airflow pattern on tail

because of various in-flight manoeuvres. In this article

we will restrict ourselves to anti-clockwise rotating main

rotor helicopters, in which tail rotor thrust is towards the

right side.

2. Effects on Anti Torque System. Some

important effects on anti-torque system :-

a. Tail rotor thrust is the result of application

of anti-torque pedal. If the thrust is more


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than required to counter main rotor, required

to counter main rotor, required to counter

main rotor, the helicopter will yaw or turn to

the left about the vertical axis and vice

versa. The environments in which

helicopters operate vary the thrust

requirement, because of constantly changing

wind direction and velocity due to main

rotor vortices.

b. Certain relative wind directions are more

likely to cause tail rotor.

c. Manoeuvring helicopter at low altitude andhigh power setting, needs a greater tail rotor

thrust as measured by its cross-wind /

sideward flight capability. Higher sideward

flight capability translates directly into

greater protection from LTE.

3. Conditions / Manoeuvres Conducive to LTE

a. Low Airspeed.


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b. Cross weight and density altitude.

c. Power droop.

d. Right sideward taxi.

e. Right hover turn.

f. Left sideward taxi.

g. Preventive Measures

(1) Plan approaches to avoid high rates of

descent that in turn, will require sharp

power pulls to stop.

(2) When manoeuvring between hover

and 30 knots :-

(a) Avoid tailwinds. If loss of translational lift occurs, it will

result in an increased high

power demand and an

additional anti-torque

requirement.

(b) Avoid out of ground effect

(OGE) hover and high power


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demand situations, such as low

speed downwind turns.

(c) Be aware that if a considerable

amount of left pedal is being

maintained a sufficient amount

of left pedal may not be

available to counteract an

unanticipated right yaw.

(d) Stay vigilant to power and wind

conditions.

(e) Avoid right pedal turns at low

altitudes and high powersettings.

h. Recommended Recovery Techniques

(1) If a sudden unanticipated right yaw

occurs, pilot should perform the

following :-

(a) Apply full left pedal and

simultaneously move cyclic


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forward to increase speed. If

altitude permits, reduce power.

(b) As recovery is effected, adjust

controls for normal forward

flight.

(c) If the rotation can not be

stopped and ground contact is

imminent, an autorotation may

be the best course of action.

The pilot should maintain full

left pedal until rotation stops,

then adjust to maintain heading.


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GROUND CUSHION

1. Gen. GM in free air the resistance opposing the

mov of air being induced to flow down ward from the

MR disc is simply resisted by the surrounding air.

2. However in hover close to gr, the gr will also resistthe induced flow this addl resistance being max when

hovering just above the surface. The down wash from

rotor is deflected by the gr into a flow radiating out word

from the hel and is dissipated against the surrounding air.

3. Now at the same time same down wash is deflect

inward underneath the hel belly. This is brought to rest

forming a dome of stagnate air, or slow moving air. This


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done of stagnate air dead air or gr cushion slightly inc pr

and causes a reduction in induced flow.

4. Comparison between Hovering in and Out of

Ground. The dir of flow relative to blade changes,

increasing AOA thereby the same AOA can be maint in

gr eff (IGE) with less coll pitch and power than req for

OGE. This reduction is power is possible because of

reduction in rotor drag.

a. Factors Eff the Gr Cushion

a. Ht of Hel. Hovering above the gr (the eff

disappears at a ht equal to approx there

quarter of disc).b. The Nature of Gr. Rough gr

dissipates the cushion.

c. The slop of the Gr. Produces an

uneven gr cushion.

d. Wind. The cushion is displaced down

wind.


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GROUND RESONANCE

1. Ground resonance can be defined as being a

vibration of large amplitude resulting from a forced or

self-induced vibration of a helicopter in contact with or

resting upon the ground. The pilot will recognize ground

resonance from a rocking motion or oscillation of the

fuselage and, if early corrective action is not taken, the

amplitude can increase to the point where it will be

uncontrollable and the helicopter will roll over.

2. Causes of Ground Resonance. The initial

vibration which causes ground resonance can already is

present in the rotor head being fore the helicopter comesinto contact with the ground. Ideally the disc should have

its centre of gravity over the centre of rotation, but it for

any reason its position is displaced, a wobble will

develop, the effect being similar to an unbalanced fly-

wheel rotating at high speed. Ground resonance can also

be induced by the undercarriage being in light contact

with the ground, particularly if the frequency of


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oscillation of the oleos and/or tyres is in sympathy with

the rotor head vibration.

a. Rotor Head Vibration. Rotor head

vibration can be caused by:

(1) Blades of Unequal Weight or

Balance. Blades should be

correctly weighed and balanced

during manufacture, but flight in icing

conditions can cause imbalance due to

the uneven accumulation of ice on the

rotor blades. Moisture absorption or

blade damage can also be a caused ofimbalance.

(2) Faulty Drag Dampers. With a three

bladed rotor system the blades should

be equally spaced 120 apart. If a

damper is sticking or is allowing

uneven spacing of the blades, the

centre of gravity of the rotor will be


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displaced away from the axis of

rotation.

(3) Faulty Tracking. A rotor, which is

greatly out of track, may set up an

unbalance condition, which will be

transmitted through the helicopter.

This type of imbalance usually results

in nothing more than a rough

helicopter and a beat in the cyclic

stick. However, if enough track

imbalance exists, it is possible that a

combination of factors may beencountered that would result in

ground resonance being induced.

b. Fuselage Vibration. Fuselage vibration

can be caused by:

(1) Mislanding aggravated by continuous

lateral movement of the cyclic stick.


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(2) A taxying take-off, or run-on landing,

over rough or uneven ground.

(3) Incorrect or unequal tyre pressures.

3. Recovery Action. The more appropriate of the

following actions must be taken:

a. Take-off immediately if take-off rotor rpm

are available. Rotor rpm should always be

maintained in the operating range until the

final landing has been completed.

b. Shut down immediately if take-off Rrpm are

not available, or if take-off is not

practicable; i.e. lower pitch lever, reducepower, apply rotor brake and wheel brakes


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RECIRCULATION

1. Introduction. Not necessary that whenever

helicopter is hovering near the ground that you can get

ground effect. As we know that some of the factors

which restrict in forming the component of dead air

underneath the aircraft. So instead of assisting the

helicopter to hover, whenever such situation arises more

power is required to hover IGE than OGE.

2. Recirculation. Whenever a helicopter is

hovering near the ground, some of the air passing

through the disc is recirculated and it would appear that

the recirculated air increases speed as it passes throughthe disc a second time. The local increase of induced

flow near the tips give rise to a loss of RT. Some

recirculation is always taking place but over a flat, even

surface the loss of rotor thrust due to recirculation is

more than compensated for by the ground cushion effect.

If a helicopter is hovering over tall grass or a similar

surface the loss of lift due to recirculation will increase


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and, in some cases, the effect will be greater than the

ground cushion.

3. When this situation arises, more power and

collective pitch is required to hover near the ground than

to hover in free air. Recirculation will increase when anyobstruction on the surface, or near where the helicopters

is hovering, prevents the air from flowing evenly away.

4. Correction for Tail Rotor Drift. Main rotor

thrust is offset to produce a side thrust to correct for tail

rotor drift. This is achieved either automatically through

design or by the pilot tilting the disc in required direction.


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5. Implication. Implication of TR drift can be

well understood if we carefully monitor the flying


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FLARE AND ITS EFFECTS

1. Definition. Tilting the disc in the opposite

direction of helicopter flight.

2. Thrust Reversal. Figure 1a represents a

helicopter in normal forward flight and figure 1brepresents a helicopter in forward flight with the pilot

executing a flare. By tilting the disc away form the

direction in which helicopter is traveling the thrust

component of the TRT will now act in the same direction

as the fuselage parasite drag, causing the helicopter to

slow down very rapidly. The fuselage will respond to this

rapid deceleration by pitching up, because reverse thrust

Parasite

Drag

Total Rotor

Thrust

Thrust

Total Rotor

Thrust

Thrust

Parasite

Drag

Pitch

Up


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is now being applied while parasite drag decreases. If the

pilot takes no corrective action, the disc will tilt back

further still, causing an even greater deceleration.

3. Increase in TRT. Another effect of tilting the disc

while the helicopter is moving forward is to change the

airflow relative to the disc. As we know, a component of

the horizontal airflow, due to the helicopter forward

movement, is passing through the disc at right angles to

the POR in the same dir as induced flow. When the disc

is flared, a component of the horizontal airflow will

oppose the induced flow and the changed direction of the

airflow relative to the blade will cause an increase inangle of attack and therefore an increase in TRT (Figure

2a, 2b).

Rotor Thrust

RA

FTip Path

Plane

b Flare

Induced

Flow

Component

of Horizontal

Airflow

Vr

Vr

RAFTip

PathPlane

Rotor

Thrust

a Forward Flight

Component

of Horizontal

Airflow

Induced

Flow


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If no corrective action is taken, the hel will climb.

Collective pitch must therefore be reduced if constant

height is to be maintained.

4. Increase in Rotor Rrpm. Unless power is

reduced when collective pitch is reduced to maintain

height, the Rrpm will rise. They will also increase rapidly

in the flare for two other reasons :-

a. Conservation of Angular Momentum. The

increase in TRT will cause the blades to

cone up. The radius of the blades CG from

the Axis of rotation (AOR) decreases andthe blades rotational velocity will

automatically rise. Power must therefore be

reduced to keep Rrpm constant.

b. Reduction in Rotor Drag. Rotor drag

is reduced in the flare because the total

reaction moves closer, towards AOR as a

result of the changed direction of the RAF.


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In figures 3a and 3b, lift and drag vectors

have been used to position the total reaction

and to show that in the flare, even allowing

for a work lift / induced drag ratio as a result

of a greater angle of attack, forward so

reducing the rotor drag. Since engine power

is being used to match the rotor drag for a

given Rrpm, any decrease in the drag will

require a reduction in power to maintain

constant rotor rpm.


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Axis of Rotation

DragLift

Total

Reaction

Rotor Drag

Rotor Thrust

Tip Path Plane

RAF

a. Forward Flightb. Flare

Axis of Rotation

Drag

LiftTotal

Reaction

Rotor Drag

Rotor Thrust

Tip Path Plane

RAF

Axis of Rotation

Drag

Lift

Total

Reaction

Rotor Drag

Rotor Thrust

Tip Path Plane

RAF

a. Forward Flightb. Flare

Axis of Rotation

Drag

LiftTotal

Reaction

Rotor Drag

Rotor Thrust

p Path Plane

RAF


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DANGEROUS CURVES

1. The establishment of fully developed autorotation,following an engine failure, will involve a loss of height.

This loss of height will vary, depending upon the air

speed at the time of the engine failure. In the hover, or at

low forward speed, the loss of height necessary to

establish full autorotation will be considerable as it will

be necessary to lower the lever fully to restore rotor rpm.

At high forward speed it may be possible to flare the

aircraft before lowering the lever, which will help to

restore the rotor rpm and may even result in a gain of

height. However, as speed is reduced it will be necessary

to lower the lever to prevent the rotor rpm from falling

again. If the engine failure occurs at or about the

minimum power speed some height will be lost in

establishing full autorotation, but it will be much less

than that lost when in the hover. For these reasons the

helicopter should not be kept in the hover between

approximately 10 ft and 400 ft AGL for any period


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longer than is absolutely necessary, and flight within the

avoid area should be kept to a minimum. The relevant

aircraft Aircrew Manual should be consulted for specific

techniques. Fig 1 shows a typical helicopter avoid area

diagram.

Air Speed (knots)

10

20 50

400

200

300

100

500

10 40 60

Avoid

AreaHeight

(Feet)

30

Figure 1 Typical Autorotation Avoid Area


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BLADE SAILING

1. A condition known as blade sailing can occur

when the rotor is starting up or slowing down in strong

wind conditions, particularly if the wind is strong and

gusting. With hel facing into wind, the adv blade

experiences an inc in lift and will flap up excessively due

to the low centrifugal force, reaching its max retreating

side it experiences a sudden loss of lift and will flap

down rapidly, flex and reach its lowest position to the

rear of hel i.e. over the tail cone. There is a danger of the

blade striking the tail cone. Now due to poor stick

response and low Rrpm, it is almost impossible to controlblade sailing. The effects can be minimized by following

methods :-

a. Displace the stick fwd and slightly into

wind.

b. Face the hel slightly out of the wind so that

lowest pt of blade passes sideways.


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c. Slow down rotor quickly by applying brakes

on shut down.

d. During start up engage rotor at a faster rate

than normal.


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LIMITS OF RPM

1. Gen. We know that max Rrpm are ultimately

governed by design considerations, but in prac may be

restricted by such factors as max eng rpm in the piston

engine or Txmn limitation in gas turbine eng.

2. Before discussing the limits of Rrpm let us revise

certain terms :-

a. Lift. It is the force produced by an aerofoil

that is perpendicular to RAF.

b. Centrifugal Force (CFF). It is the

force which tends to pull a rotating body

away from the AOR.c. Centripetal Force. It is the force that

counter acts centrifugal force by keeping an

object a certain radius from the axis of

rotation.

3. Consider when the rotor blades are at rest they

drop due to their weight and span. When the rotor system


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begins to turn, the blade starts to rise from the static posn

because of the centrifugal force.

4. As the hel develops lift during take off and flight

the blades rise above the straight out posn and assume a

coned posn amount of conning depends on Rrpm, gross

wt and G forces experienced during flt.

5. Excessive conning can cause undesirable stresses

on the blade and a decrease of total lift because of a

decrease in effective disc area. Let us consider the

resultant of lift and centrifugal force.

6. The vertical force is lift produce when the bladeassume positive angle of attack. The horizontal force is

caused by the centrifugal force due to rotation. Since one

end of blade is att to the rotor shaft it is not free to move.

The other end can mov and will assume a posn that is the

resultant of forces acting on it, and the blade posn is

coned as a resultant.


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7. Limits of Rrpm. As the centrifugal action

through Rrpm gives a measure of control of the conning

angle, provided the Rrpm are kept above the specified

minimum, the conning angle will always be within the

safe operating limit. There will always be upper limits to

the Rrpm due to engine or transmission consideration and

end loading stresses where the blade is att to the rotor

head. Rrpm limitation will be found in the relevant Air

crew manual.


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ADVANCE ANGLE

1. As we have seen that blade flapped position will

always be 90 degree out of phase with control orbit and

blade reaches its highest and lowest point 90 degree later

than where it experiences the maximum and minimum

increase and decrease of cyclic pitch so if the control

orbit tilts in the same direction as cyclic stick is being

moved and as a result of change in cyclic pitch the rotor

disc tilts 90 degree out of phase with the control orbit,

then the disc will always be tilting 90 degree ahead of the

cyclic stick application. Unless compensated by some

way, moving the cyclic stick forward will cause thehelicopter to move sideways. This can be over come by

following.

a. To arrange the blade to receive the max

alteration in cyclic pitch change 90 degree

before the blade is over highest and lowest

point over the control orbit.


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b. The control orbit tilts 45 degree out of phase

with stick movement so 45 degree advance

angle is needed only to compensate for

phase lag.


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+2 -2Blade

High

Blade

Low

AdvanceAngle

90

O

O

Stick Right

Figure 45 90 Advance Angle

Control Orbit

Tilt Axis

+2

-2

Blade

High

Blade

Low

Advance

Angle

O

Stick Right

Figure 46 45 Advance Angle

Control Orbit

Tilt Axis

+2

O

O

O

45 45

Control Orbit

Tilt Axis 45


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POWER REQUIRED

Parasite Power

1. Parasite power is the power required overcoming

the drag of the fuselage when the helicopter is in straight

and level flight. If the drag is calculated for a given speed

and that speed is doubled, the drag will increase four

times but the power required to overcome this rise in

drag will increase eight times. The curve produced by

plotting parasite power against forward speed will have a

zero value when the helicopter is in the still-air hover but

will rise progressively steeply as speed increases (see

Fig 1).

= TotalPower

Required

Parasite

Power

Induced

Power

RotorProfile

Power

Power

(Drag xVelocity)

TAS

Figure 1 Power Curves


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Rotor Power

2. If a helicopter is hovering in still air, the total rotor

thrust being produced will be equal to the weight. Within

limits, depending upon the type of helicopter, this total

rotor thrust can be produced from a wide variation of

collective pitch settings and rotor rpm (Rrpm), but the

drag, and therefore the power, will vary with each

combination, and only one combination will minimum

rotor drag. The power needed to drive the rotor can

therefore be considered from two aspects:

a. The power related to a variation in the value

of pitch or drag coefficient (CD0: this isrotor profile power.

b. The power related to a change in pitch or Cd

for a constant rotational velocity; this is

induced power.

Rotor Profile Power

3. The Cd of a rotor blade will vary with collective

pitch setting, but will remain constant and have its


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minimum value when the collective pitch is minimum.

For a given will vary only with the changes in Rrpm and

air density. However, whenever the main rotor is turning,

ancillary equipment, associated drive shafts and the tail

rotor will also be absorbing power, the power absorption

varying mainly with the velocity of the main rotor. All

these power requirements are included in calculating

rotor profile power so that rotor profile power can be

defined as being the power required to maintain a given

Rrpm when the collective pitch is minimum and to

overcome the drag of ancillary equipment, associated

drive shafts and the tail rotor the rotor profile powercurve will start at a position on the vertical axis o the

graph at Fig 1 depending upon the Rrpm selected and the

air density. Assuming a constant value of CD, as forward

speed increases the power required maintaining this

Rrpm will increase. This increased power requirement is

because in forward flight the increase in drag of the


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advancing blade will be greater than the decrease in drag

of the re-treating blade.


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53

PART II

METEOROLOGY

1. MET ORG / OBSERVATORIES IN

PAKISTAN

2. LAYERS OF ATMOSPHERE

3. WINDS AND ITS CAUSES

4. LOCAL WINDS

5. VISIBILITY

6. TURBULENCE

7. PRECIPATION8. THUNDER STORMS

9. FOG FORMATION

10. ICE FORMATION

11. WEATHER IN THE MOUNTAINS

12. EFFECT OF FLYING CONDITIONS IN LOW

PRESSURE AND HIGH PRESSURE


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MET ORG / OBSERVATORIES IN PAKISTAN

1. Introduction. Met is the branch of science

which deals with the earths atmosphere and physical

process. Imagine that even the most modern ac today are

dependent on this branch of Aviation. Every military

mission planning includes deliberate MET planning

including minutest details to ensure success of mission.

2. Function of MET Organizations. These are

designed to fulfill 2 main functions :-

a. Record and report information of past and

present weather.

b. To forecast future development of weatherthrough :-

(1) Network of weather observations.

(2) System of rapid communication for

collection and dissemination of

information.

(3) A central forecast office.


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(4) System of local Met office in assisting

provision of information.

3. Main Types Observatories in Pakistan. There

are 6 different types of main observatories available in

Pakistan:-

a. Surface observatory.

b. Pilots balloon observatory.

c. Radar wind observatory.

d. Radio sonde.

e. Radar station.

f. Satellite weather picture.

4. Categories of Met Offices. There are 3categories, details as follows :-

a. Cat I

(1) Established at main operational

airfield.

(2) Maintain a constant forecasting

watch.


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(3) Supply of Met information, briefing

to Avn personnel.

(4) Provides guidance to Cat II & III

offices of Met.

b. Cat II

(1) Situated close to main operational

airfd.

(2) Maintain restd watch. (Upto 100

units).

(3) Prepare enroute wx forecast under

guidance of Cat I.

c. Cat III(1) Situated at a base which does not have

routine operational commitment.

(2) Supply Met info on request basis

received from cat I/II.

(3) Brief / Debrief aircrew on operational

occasions.


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(4) Maintains record of local as well as

out station weather information.

5. Conclusion.The weather info is provided to help

air crew to plan and operate with max efficiency. To

extract the fullest benefit from met service one must

acquire a good working knowledge of the all physical

activities atmosphere.


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LAYERS OF ATMOSPHERE

1. Gen. The atmosphere in which we are living

consists of various gases, i.e., 78% Nitrogen, 21%

Oxygen and 1% other gases. Besides these gases some

other items like water vapors are also present in the

atmosphere and play a significant role in formation and

development of various weather phenomena. Particles of

dust smoke etc. are also available in the atmosphere.

2. Let us see that how far particles can go, what is

temperature and height and how the weather in the upper

layer would be.

3. Division of Atmosphere. Atmosphere isdivided into following layers:-

a. Troposphere. It extends from ground to

6 miles where temperature is constantly

decreasing with the increase in height.

b. Stratosphere. It extends from 6 miles

to 22 miles where temperature is initially

decreasing and then increase with height.


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c. Mesosphere. Extends from 22 miles to

50 miles. Temperature initially increases

with height and then reduces to 130C.

d. Thermosphere. Height is 50 to 70 miles

where temperature constantly increases with

increase in height.

4. Conclusion.Besides gases atmosphere also

contains a small but very variable amount of invisible

water vapor which causes many weather changes like

formation of clouds fog, rain and snow.


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WINDS AND ITS CAUSES

1. Gen. We in Pakistan have a variety of terrain,

starting from snow covered peaks to the barren deserts

and long coastal areas. We being army pilots have to

operate over every type of terrain. The Wx phenomenon

are going to be different at different types of terrain.

Every terrain has its own peculiarities. So we can not

generalize the weather throughout the country.

2. There are certain local Wx phenomenon in certain

parts of the country e.g. if you fly in the Northern Area in

the morning the winds are going to be different than if

you plan in the afternoon. The same way there are certainvalleys where you experience generally isolated build up

and strong wind through out the year. The same way if

you are close to the coastal areas the wx phenomenon is

going to change in the morning and in the afternoon.

3. Causes of Wind

a. Pressure variation.

b. Rotation of the earth.


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c. Diurnal variation of surface wind.

d. Different land surfaces.

4. Pressure Variation. At any given level in the

atmosphere the barometric pressure is everywhere the

same, there will be no isobars, no pressure gradient, and

consequently no wind. When there is a pressure gradient,

however, there will be an initial tendency for the air to

flow across the isobars in the direction from high to low

pressure, but the resultant flow will be almost parallel to

the isobars.

5. Rotation of the Earth. We have seen that when

air begins to move horizontally it blows from high to lowpressure, but that after the motion has continued for some

time the flow tends to be along the isobars, with low

pressure on the left in the northern hemisphere. This

apparent deviation to the right of the direction we should

expect the air to follow is a relative effect, being due to

the fact that we measure the motion relative to the

rotating earths surface.


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6. Diurnal Variation of Surface Wind. There is a

fairly regular change of wind in each 24 hours: it veers

and increases by day, reaching its greatest strength in the

afternoon, and then backs and decreases, with a

minimum strength about dawn. This sequence is known

as the diurnal variations of wind. The normal diurnal

variation of wind over land in the surface frictional layer

in the northern hemisphere may be summarized as

follows :-

a. Surface winds normally veer and increase by

day, but back and decrease at night.

b. Above the surface, at say 2,000 ft., the windnormally backs and decreases by day, but

veers and increases at night.

7. The diurnal variation of surface wind over land is

especially significant to aircrew because of the important

part it can play in helping the formation of very low

cloud or fog at night and leading to its dispersal by day.

The diurnal effect over the sea is very small.


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8. Different Land Surfaces. layer owing

mainly to the nature of the local topography, abnormal

surface winds are regularly observed in particular

localities. Land and sea breezes are a case in point: other

important examples are :-

a. Valley winds.

b. Katabatic winds.

c. Anabatic winds.

9. Conclusion. Wind are of major importance to

aircrew. At every take off and landing a pilot must take

in considerations the direction, speed and its gustiness for

safe operations.


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LOCAL WINDS

1. Gen. Winds at the surface is normally closely

related to the geostrophic wind in the free atmosphere

above friction later owing mainly to the nature of the

local topography, abnormal surface winds are regularly

observed in particular localities.

2. Types of Local Winds

a. Land and Sea Breeze. Consider two

columns of air initially of the same

temperature and with same atmospheric

pressure at pt C and D. Imagine that column

AC is over land and BD on Sea and the Sunjust arisen. Soon the air over the land will

become warmer than that over the sea. The

air in column AC will expand and so the qty

of air above A will increase and hence the

pressure at A will become greater than B.

Air will start to blow from A to B. As soon

as this happens, the amount of air above C


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will be depleted and above D augmented, so

the surface air pressure at C will begin to

fall, and that at D to rise. With lower

pressure now at C than at D, air will begin to

flow from sea to land, a sea breeze will set

in.

b. Valley Winds. A wind blowing against a

mountain has more speed as in a river large

rock blocking the stream. If a barrier is

broken by a narrow gap the stream flows at

increased speed through the channel.

Similarly if a mountain barrier is broken bya valley, the wind tends to blow along the

direction of valley at a speed greater than

neighboring region on other side. In valley

the strongest winds are likely to be along the

general dir of valley.

c. Katabatic Winds. When land is cooled by

radiation during a clear night, air in contact


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with the ground is also cooled and in

consequence its density increases. If the land

is sloping, the denser air tends to flow down

the slope and the air movement is known as

a katabatic wind.

d. Anabatic Winds. The opposite

phenomenon of air moving up the slopes of

valley when the land is warmed on a sunny

day is known as an anabatic but effect is

generally slight.

3. Conclusion.The local winds phenomenon is

significant mostly in mountainous terrain and coastalareas. So when planning in to a mountainous area, you

must consider the prevailing phenomena and must

consult the pilots who have operated in that area.


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VISIBILITY

1. Definition. Met visibility is defined as the

greatest horizontal distance at which an objects can be

seen and recognized by an observer with normal sight

and under conditions of ordinary daylight.

2. Factors Effecting Visibility. The distance at

which one can see the objects by day, or lighted objects

at night depends on many factors, of which the most

important are :-

a. Geography i.e. presence of surrounding

objects.

b. Amount of smoke mist or haze in the air.c. Color, brightness and size of the object.

d. Color and brightness of the background.

e. Sensitivity of the observers vision.

f. Transparency of any window, windscreen,

etc, through which the observer looks.

3. Measurement of Visibility by Day. A met

observer estimates horizontal visibility with the help of


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suitable objs at known distances. Visibility objs should

be dark in color with a lt back gr. For distances over 10

miles, hills, are usually the only suitable objs.

4. Causes of Poor Visibility. It is due to :-

a. Solid particles such as dust, sand, smoke or

soot.

b. Visible moisture in the form of cloud,

precipitation, spray, fog or mist, consisting

of water droplets or ice crystal.

5. Effect of Sun or Moon on Visibility

a. The distance at which objects can be

recognized may also vary with the directionof viewing in relation to the position of the

sun or moon.

b. Observe having his back towards the sun

would be able to see at longer dist then

another observer who is looking towards the

sun.


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c. Opposite happens in case of moon light as

the visibility increases towards the

moonlight and away from it.

6. Oblique Visibility is Effected By. There are

few limiting factors for the observer sitting in the ac or in

other words visibility from the ac is hampered due to

some factors which are :-

a. Curvature of the earth.

b. Mist / Fog above the surface of the earth.

c. Height of the observes.

7. Descriptive Terms. Terms like good,

moderate and poor are usually applied to definite rangesof visibility, which are :-

Visibility Description

Less than 44 Yards Dense Fog

44-220 Yards Thick Fog

220-440 Yards Fog

440-1100 Yards Moderate Fog

1100-2200 Yards Mist


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1.0 5.0 NM Poor Visibility

2.0 5.0 NM Moderate

5-11 NM Good Visibility

11-22 NM Very Good

Over 22 NM Excellent

Visibility

8. Conclusion. Despite improvements in blind

flying equipment, visibility remains one of the most

important weather items affecting air operations. While

planning air operation all the factors effecting visibility

must be catered for to ensure safety of men and materiel.


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TURBULENCE

1. Gen. It has assumed increasing importance to all

pilots during the last few years because of high speed

aircraft. Previously it had meant no more than physical

discomfort. Now a days the pilot must bear in mind the

structural stresses placed on the aircraft by the

turbulence.

2. Types of Turbulence

a. Clear Turbulence. This type of

turbulence is not necessarily associated with

clouds and is often difficult to forecast. It

occurs at high altitude and its height band isvery shallow.

b. Thermal Turbulence. The steep lapse

rate result from thermal effect. These are

found in unstable air. These are usually most

noticeable at low altitude.

c. Ground Turbulence. These type are

specially noticeable in the leeward of hills,


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woods and large building. These are likely

to be severe, and strong downdraft may

endanger ac flying very close to ground.

These types may produces a thin layer of

clouds.

d. Turbulence due to Mountains. In

addition to ground turbulence and irregular

vertical convection current over mountains,

the disturbance of the airflow by a mountain

range may upset a regular flow pattern

waves. These may be experienced to a

considerable height above the crest of amountain and may persist for many miles

down wind. Pilot will experience this most

when he is flying leeward side of mountain.

Pilot should be more careful while flying in

these areas.

3. Effect of Speed. Speed is the most important

factor when flying in turbulence. As an ac enters updraft


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the loading on the wings suddenly increases until upward

motion of the ac is adjusted to that of surrounding

updrafts. If airspeed is low the effect of turbulence will

be less.

4. Flying Technique Turbulent Weather. The

main req is to keep the ac on a fairly even keel by the

moderate use of cons. Aim should be to allow the ac to

ride the bumps rather than to fight them. Rough handling

only aggravate the stresses already imposed by

turbulence.

5. Conclusion.Being an aviator one should

understand enough about the weather to unable air crewto asses the restrictions imposed by it.


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PRECIPATION AND ITS FORMS

1. Gen. The onset of precipitation is usually

accompanied by rapid lowering of the cloud base and by

reduced visibility from the cockpit. The weather

deterioration of this kind poses greater threat to avn

operation.

2. Types of Precipitation

a. Rain. Rain consists of water drops of

appreciable size up to about half an inch in

diameter. In temperate latitudes rain usually

originates in cloud as aggregates of ice

crystals, which melt on falling below thefreezing level and turn into raindrops. Hence

it is quite usual of aircraft to encounter snow

when flying in temperate regions at an

altitude where the temperature is near or

below 0C., although the precipitation

reaching the ground may be in the form of

rain.


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b. Hail. Hail consists of small lumps of ice

and is too well known to need further

description. Hailstones smaller than golf

balls have fallen in Pakistan on rare

occasions.

c. Snow. The ice crystals in cloud above

the freezing level may grow and interlock

until they become too large to be supported

by the ground without melting if the air

temperature below the cloud is sufficiently

low.

d. Sleet. Sleet is defined as rain and snowfalling together, or snow melting as it falls.

e. Drizzle. Drizzle consists of water

droplets so small that their individual impact

on a water surface is imperceptible. It is

often associated with mist or fog, and

usually falls from thin layer clouds.


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3. Effect of Precipitation on an Ops

a. Effect on Rain

(1) Reduction in fwd visibility.

(2) Tendency of skidding on corners or

turns.

(3) In T/O, fwd flight, reduced fwd

visibility.

(4) Risk of icing.

(5) In landing heavy rain will affect radio

nav aids i.e. GCA Eqpt.

(6) Damage to horizontal stabilizers / tail

rotor during landing when tyres throwup water.

(7) Efficiency of brake.

b. Effect of Hail

(1) Damage to airframe of engine

(canopy) fwd contours.

(2) Greater speed greater is damage.


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(3) By flying above or below freezing

level chances are minimized.

c. Effect of Snow. Generally create

problems from reduce visibility and

difficulties in landing, T/O, and taxiing.

(1) Before T/O remove moisture and

snow.

(2) During taxing brakes less effective.

(3) May blow up in hover and whiteout.

(4) Helicopter may slip on slope.

(5) In landing hel may sink in and

resonance is experienced.4. Conclusion. Precipitation produces lot of

flying hazards. All pilots must know their types and

condition surrounding them.


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THUNDER STORMS

1. Gen. Thunder storms produce the most severe

type of weather known to mankind. Tornadoes with

winds reaching 350 mph, hailstones the size of baseballs,

and extreme turbulence can result from thunder storm

formation. A thunderstorm is invariably produced by a

cumulonimbus cloud and is always is accompanied by

lighting and thunder. Thunderstorms are a hazard for all

types of flight operations since top of cumulonimbus

clouds may reach as high as 75000 feet.

2. Factor Effecting Formation of Thunder Storm.

Several factors must exist for the formation of athunder storm. First, for the formation of the cumulus

cloud and for its continuing build-up, some sort of lifting

action may be orographic, conventional, or frontal.

3. Stage of Thunderstorm

a. First Stage. There are three stages to the

development of a thunderstorm. The first

stage is known as the cumulus stage. The


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main feature of this stage is the cumulus

could and the updraft which may extend

from near the earths surface to several

thousand feet above visible cloud tops.

Water droplets are vary small but grow into

raindrops as the clouds build upward. Many

times the raindrops remain in the liquid state

even above the freezing level. These rain

droplets are suspended by the currents

within the clouds.

b. Second Stage. The second, or mature

stage is the most intense phase. It begins asrain begins to fall at the earths surface.

Raindrops and ice particles, by this point,

have grown to such a size that they no

longer can be supported by the updrafts. The

mature stage occurs approximately 10 to 15

minutes after the cloud has been built

beyond the freezing level in the atmosphere.


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Occasionally, during the mature stage, a

cloud may build as high as 50000 to 60000

feet, but 25000 to 30000 feet is the norm.

Severe up and downdrafts occur in the

mature stage. As the raindrops fall, they pull

air with them and create downdrafts that

may exceed 2500 feet per minute. This

causes gusty winds at the surface as the

downdrafts strike the earth and spread out.

c. Third Stage. The dissipating stage is

characterized by the collapse of the

cumulonimbus cloud. Downdrafts continueto develop and spread vertically and

horizontally while updrafts weaken and

finally dissipate completely. Soon the entire

thunderstorm becomes an area of

downdrafts. Rain decrease, then ceases, and

the thunderstorm begins to dissipate. The

top of the thunderstorm, at this stage, begins


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to develop the characteristic anvil

appearance with the point of the anvil in the

direction of the prevailing winds.

4. Conclusion.The hazards that exist with thunder

storm activity are not confirmed just to the storm area

itself. But pilots flying near the storm area can also

encounter its advance effects.


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FOG FORMATION

1. Gen. Fog is basically a cloud which is very near

or touching the Earths surface. It consists of small water

droplets or ice crystals suspended in the atmosphere.

They are very small (droplets) to see with the naked eyes.

But they are so numerous that visibility is reduced. In

short we can say it is minute droplets of water or ice

suspended in the air with no visible downward motion

and visibility less then 1100 yards. Various forms of fog

are :-

2. Formation of Fog Depends Upon. For the fog

formation following are the pre-requisites:-a. High Relative Humidity. The high

humidity necessary, for fog formation can

occur in distinct way:-

(1) When air is cooled to its dew

point.

(2) When the moisture is added to

its dew point.


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b. Wind. Light surface wind is also

necessary wind provides a mixing action. If

no wind is present the fog will likely be

shallow and also to the ground, strong winds

are not conducive to fog formation. It tend

to break up the fog layer.

c. Condensation Nuclei. Condensation

nuclei, such as smoke dust and salt particles

suspended in the air, provide a base around

which moisture condenses. Our country is

quite rich in this regard and sufficient nuclei

would be present to permit fog formation.The amount of smoke particles and sulfur

compound in the vicinity of industrial areas

is pronounced. In such area persistent fog

may occur.


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3. Types of Fog

a. Radiation Fog. It forms as the earth

rapidly loses is heat on clear nights.

b. Advection Fog. Is formed when warm

moist air flows over a cold surface this type

is found mostly along the coastal regions,

where temperature of land and water differs

widely.

c. Up Slope Fog. When moist air flows up

hill. As it rises the temperature. Drops

through adiabatic cooling and by

evaporation.d. Steam Fog. When cold air flows over water

which is much warmer than air.

e. Sea Fog. It happens when moist air mass

usually of tropical region moves slowly over

cooler seas.

f. Valley Fog. During evening, cold dense air

will drain down into low areas or valleys.


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g. Ice Fog. When air near surface becomes

saturated in extremely cold region fog will

form.

h. Smoke Fog. Around industrial city.

4. Fog Dissipation. Fog would tend to dissipate

under following conditions :-

a. Dec in Relative Humidity. It tends to

dissipate when relative humidity decreases.

During process water droplets evaporates or

ice crystal sublimate, and moisture is no

longer visible.

b. Strong Wind. As it is stated above thatstrong wind mix the cool saturated air at the

surface and warms air of the atmosphere.

c. Heating Up of Atmosphere. Air which is

heated as it flows down slope or by day time

solar radiation evaporates fog. Most of it

dissipate after sunrise.


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5. Conclusion.Fog though is one of the cause of poor

visibility but posses greater damages since it hides the

mother earth completely leaving air crew with no contact

with it. Knowledge about various types will help air crew

in negotiating it safely.


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ICE FORMATION

1. Gen. Ice would be a serious hazard if it forms on

an aircraft in flight. With modern aircraft, icing problems

rarely arise. If icing regions are encountered, they can

often be crossed in few minutes or can be avoided by

changing altitude, while on different aircraft deicing

equipment are fitted. Nevertheless, ice formation may

still sometimes cause a serious loss of aircraft

performance. A knowledge of the types of ice and their

modes of formation is necessary if aircrew are to take

most effective option in these circumstances.

2. Types of Ice Formationa. Hoar Frost. It occurs in clear air and is

easily recognized as light crystal deposits. It

occurs below 0,C temperature.

b. Rime Ice. It is rough white ice which

forms on the windward side of trees and

other exposed objects. It also occurs below

0,C temp.


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c. Clear Ice. It is most dangerous ice and

consists of a transparent ice with a glassy

surface.

d. Pack Snow. It occurs on aircraft surface due

to super cooled water droplets.

3. Dedication of Icing. Early detection of icing

its necessary, as it may cause serious difficulties if timely

action is not taken. By day it is easy to detect ice by

observation of windows, LE, propellers, and aerial masts.

4. Anti Icing Eqpt

a. Thermal. Hot air from engine is led to the

surface to be protected, or the surface isheated electrically.

b. Chemical. Alcohol spray, other deicing

fluids or special non freezing oil or grease is

applied to the surface of an aircraft.

c. Mechanical. A pulsating rubber boot

may be fixed to the surface such as LE, and


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employs the mechanical mean to break the

ice by intermittent inflation and deflation.

5. Conclusion. Forecasting of icing condition

is very difficult, however endeavour should be made to a

certain its presence and take timely counter measures.


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WEATHER IN THE MOUNTAINS

1. Convection and Air Mass Stability. Stable air

mass conditions with negligible surface heating will

temper the wind deviation effect but in conditions of

unstable air mass, with strong surface heating, the

vertical air currents produced will accentuate vertical

deviation thus increasing up and down drafts. In addition,

even when a slack wind gradient persists, a strong

surface heating during the day will produce quite fresh

anabatic winds, e.g. in the Aden Protectorate mountains a

40F temperature difference between valley floor and

mountain top is quite common producingg slope winds of

15 knots in mid afternoon. This surface heating is often

not even and results in strong thermal up currents and

accompanying turbulence.

2. Cloud. Clouds of orographic origin are often

found in the mountains and, dependent on temperature,

may give conditions particularly dangerous to flight, e.g.

frost, icing etc.


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3. Fohn Effect. In stable conditions another

effect of orographic cloud is to produce Fohn effect. This

may mean that a landing platform upwind of the ridge

would not be suitable for use.

4. Always keep in mind the possibility that through

down draft or turbulence the pilot may have to break off

an approach or break away from a chosen flight path and

position the aircraft so that an escape route is always

available.

5. Whenever possible when flying near the ground

reduce speed to climbing speed. Avoid areas where down

drafts are likely to be found. Should the helicopter enter adown draft, maintain climbing speed and try to counter

the loss of height with power. If unable to do so then go

with the down draft but maintain climbing speed.

6. Anticipate the increased wind strength that obtains

near cols, crests, valleys and peaks. Where possible fly at

a safe height, within reach of a reasonable landing area,

and avoid crossing close to sheer faces. In the event of an


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engine failure make the best use of any available landing

site to ensure that, whatever else happens, the helicopter

does not roll down the mountain side.


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EFFECT OF FLYING CONDITIONS IN LOW

PRESSURE AND HIGH PRESSURE

1. General. Numerous aircraft accidents have

been caused directly due to failure to read the altimeter

correctly. Proper use of this instruments is therefore

mandatory. In flight, altimeter is one of the most

important instruments which provides information for

clearing obstructions, making low approaches, avoiding

other traffic, and hazards.

2. Height / Pressure.Pressure decreases with

increase in height. Air has weight and therefore exerts

pressure known as atmospheric pressure. At any point onthe surface of the earth, the atmospheric pressure is

equivalent to the pressure exerted by a column of air

approx. 50 miles high, and can be expressed as millibars,

pounds per square inch or as the height of a column of

mercury which that pressure would support.

3. Error. Altimeter has the following errors:-.


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a. Pressure Error. This is caused due to the

position of the static vents. Because of

disturbance in the vicinity of the aircraft

false or extra pressure is fed into the system

and as a result the altimeter gives erroneous

indications. This error is negligible except

when flying through severe turbulence.

b. Barometric Error. Suppose an

aircraft is flying at an altitude of 5000 and

heading from an area of high pressure to low

pressure. Once the aircraft is over low

pressure area it would still indicate 5000but would be actually lower, say 4000 only.

This is so because in a low pressure area the

equivalent pressure i.e. 30 in this case, will

be present at a comparatively lower height.

In this instance then, the altimeter would

over read.


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d. Temperature Error. Atmospheric

temperature and pressure vary continuously.

Rarely is the pressure at sea level exactly

29.92 or the temperature +15 deg C.

furthermore the lapse rate, for both pre-

pressure and temperature, deviates from the

standard. For instance on a warm day the air,

having expanded, is lighter in weightt per

unit volume than on a colder day, and the

pressure levels are raised. Therefore, the

pressure level where the altimeter will

indicate 10000 will be HIGHER on a warmday than it would be under standard

conditions (diagram). On a cold day the

reverse would be true, and the 10000 level

would be lower.

e. Lag Error. The altimeter may tend to lag

particularly when rapid and large changes

in altitude are made. This error called


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hysteresis or after effect will vary with the

climb or descent.

f. Instrument Error. Since the

expansion and contraction of the wafer stack

are greatly magnified, it is impossible to

avoid magnifying minute irregularities.

g. Blockage Error. Should the static tube or

vents become blocked, pressure within the

case will remain constant and the altimeter

will continue to indicate the height at which

it was blocked. After breaking the glass of

the VVI it will give indications but with a 6-9 seconds lag.


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PART III

NAVIGATION

1. DEFINITIONS

2. CHOICE OF HEIGHT TO FLY

3. MAP PREPARATION

4. NAVIGATION IN MOUNTAINOUS

TERRAIN POINTS TO REMEMBER

5. PRE-REQUISITES FOR AERIAL

NAVIGATION

6. PRE-FLIGHT PREPARATION FOR LOW

LEVEL NAVIGATION7. GPS


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DEFINITIONS

1. Definitions

a. Altitude. The vertical distance of a level,

a pt, or and object considered as a pt,

measured from mean sea level.

b. Height. The vertical distance of a fixed

pt above ground level or some specified

datum other than mean sea level.

c. Elevation. The vertical distance of a fixed

pt above or below men sea level.

d. Flight Level. Flt levels are surfaces of

constant atmospheric pressure related to thestandard pressure setting of 1013.2 Mb and

separated by specific pressure intervals.

e. Transition Altitude. The altitude in the

vicinity of an airfield above which 1013.2

Mb is to be set on the altimeter. At or below

the transition altitude the vertical position of

an aircraft is determined with reference to


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altitude above means sea level or height

above an airfd.

f. Transition Level. The lowest flt level

available for use above the transition

altitude. Descent must not be made below

the transition level without setting the

appropriate QNH or QFF on the altimeter.

g. QFF. QFF is the observed pressure

corrected for temperature at an airfd

elevation. With QFF set an altimeter will

read zero height at the airfd datum.

h. QNH.QNH is the observed pressure at aselected datum, corrected for temp and

reduced to a mean sea level assuming that

the atmosphere conforms to the ICAO

standards. When set to QNH, altimeter will

indicate altitude AMSL.


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CHOICE OF HEIGHT TO FLY

1. Gen. The most important pt in the technique of

Nav is the choice of height to fly. Keeping apart the rules

of semicircular height there are certain other

considerations which must be taken is account.

2. Imp considerations for Ht to Flt

a. High Gr. Ht flown above should be safe

or above safety ht specially in the inst met

conditions (IMC).

b. AC Performance. The ht selected should be

that where best performance of the ac is

aval.c. Quadrant Ht. Always plan to fly

quadrantal heights regardless of weather

conditions.

d. Wind and Weather Forecast. It is possible

to take advantage of winds to avoid adverse

winds by careful selection of ht.


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e. Oxygen Condition. Flt above 10,000 ft

requries use of oxygen an prolonged flt

above 10,000 ft is dangerous due to hypoxic

conditions. Do not fly in these conditions for

more than 30 minutes.

f. Msn Requirement. Msn requirement

may dictate to fly as low as tree top. In these

conditions fly as per proper low flying

technique.

3. Conclusion.The factors mentioned above besides

allowing air crew with the choice of height to fly ensure

safe and efficient operation, hence must be given primaryimportance.


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MAP PREPARATION

1. Gen. When a msn is assigned carryout the pre flt

procedure and select the desired map. The carryout map

prep as follow :-

2. Imp Consideration for Map Prep

a. Draw the tack.

b. Calculate the bearing and distance and

record these things on map.

c. Mark the time markers after 10 interval.

d. Mark the imp check point along the route

and measure the distance from the main tack

either side of the track..e. Calculate time between each check pt.

f. Mark the danger and prohibited areas.

g. Mark the position of radio aids and mark the

freq.

h. Check the air fd and ATC frequency of each

air fd.


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j. Check pt and time of entry in any con area,

trg area, air fd.

k. Study the map in detail.

l. Fold the map properly so that the complete

coverage of track is achieved.

3. Conclusion.The increasing complexity of air

operation using multi-navigational aids to accomplish a

mission still desire the basic navigation aid Map for

successful, preparation / playing of a sorties.


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NAV IN MOUNTAINOUS TERRAIN POINTS TO

REMEMBER

1. Gen. Navigation in mountains is comparatively

easier than that of plains due to good visibility condition

and aval of prominent land marks, important features

and ref pts. However there are chances when the

perspective is changed and features along a designated

route appear to be different and difficult to be

recognized. To over come this sit fol pts must be kept in

mind while flying in mountainous terrain.

2. Points to Remember

a. Loss of Attitude. Since helicopter inmountains will fly oftenly in valley so

normal horizon will be deprived to the pilot,

so must cross check to fly instrument

specially to A/H.

b. Sun into Eyes. While flying in valleys

there will be a number of occasions when

sun will be into your eyes which will lose


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your perception for the depth and width of

valley also denying you the visual contacts

of turns and bends. In such condition if the

height of the crest permits then climb and

clear crest line or descend down enough and

stay on the side of valley which gives you

best vision.

c. Hazards in Narrow Valley. While flying

low in valleys must be watchful for wires

and trolley cables which are normally

crossing valleys. As a generally rule never

fly below the road level. If possible flydown the valley than up a valley. If flying

up a valley do not cross a point beyond

which a 180 turn is not possible.

d. Winds. Wind condition is much more

severe and unpredictable than that of plains.

In mountains specially in valleys stay on the

lifting side i.e. In case of cross wind stay on


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the lifting / climbing wind side of the valley.

As a general rule stay on the sunny side of

the valley to get up draft.

e. Turbulence. Avoid turbulence zones

if possible otherwise try to get out as early

as possible. Once in turbulence do not flight

the controls, reduce the pitch slightly to

minimize stresses on transmission.

f. Icing. Do not fly in icing conditions if

possible avoid situations where visible icing

conditions are present.

g. White Out. Avoid entering in suchcondition. Keep looking around for different

references.

h. Low Air Density. With increased altitude

density of air decreases so the engine power

is reduced specially it has more effects on

piston engines thus giving less reserve

power. So avoid maneuvers which involve


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reserve power e.g. quick stop, steep tuns or

abrupt pulling of collective pitch./ the rotor

remains turning at same RPM so with

increased altitude, higher pitch setting is

required, which needs increased pitch setting

of tail rotor as well, limiting pedal control

and also bring main rotor retreating blade

close to stalling angle.


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PRE-REQUISITES FOR AERIAL NAVIGATION

1. Gen. An air navigational map may be defined as a

small scale representation of the earth and its culture. It

depicts the land marks and other information useful for

pilots during aerial navigation. The ability of pilots to

comprehend all the details help them to accomplish the

sorties with success.

2. Basic Principle of Aerial Navigation. While

flying four basic principles for aerial map reading should

be followed :-

a. Orientation. While reading the map,

orientate the map in a way that the north ofthe map is towards the north. Only then the

course lines on the map parallel the intended

course lines of aircraft and objects on right

and left of the course appear to the right and

left of the aircraft if it is on course.

b. Appreciation of Rate of Travel Over

Map. For appreciation of rate of travel on


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map, the speed and scale of the map are

necessary. It is worked out in pre flight

planning and subsequently in the air by

noting the elapsed time between two check

points.

c. Anticipation. Appearance of terrain

varies at different altitudes. In low flying a

checkpoint appears for a very short time. the

vision is also restricted but because of

oblique angle of sight, depth increase and

detail is blurred and landmark present

different appearance to that shown on map.From high altitude ground seen to appear to

move very slowly. In good visibility large

area can be seen and distances appear small.

With sun low position, shadows are long

causing strong contrast and emphasizing

difference in elevation. At moon when there

are no shadow, terrain appears to be flat


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broken clouds may block the view

completely thus hiding a check point.

d. Appreciation of Perspective.

3. Pre Flight Procedure

a. Review all relevant information documents.

b. Collect the meteorological forecast from

meteorological office and study the forecast.

c. Select the planning charts and maps to be

used on selected route.

d. Draw tracks and measure distances. Study

the safety heights and decide the height to

fly.e. Complete the map preparation.

f. Plan flight for alternate airfield.

4. Conclusion.Principles of aerial navigation have

been worked out over a period of time to ensure

accomplishment of successful mission in different terrain

/ type of country.


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PRE-FLIGHT PREPARATION FOR LOW LEVEL

NAVIGATION

1. Gen. Some hazards to flight, even if not entirely

new, assume a much greater importance in low level

flight, and the crew planning such a flight must take into

consideration factors which can normally be assumed

negligible otherwise. Sound and deliberate planning can

reduce the low level flight hazards to a great extent, some

tips for pre-flight preparation are discussed in succeeding

paragraphs.

2. Mission Briefing. Before a low level flight

begins, and before flight planning is started, the crewmust be given a clear brief on the mission, any restriction

on the routing, and the flying limitations to be observed.

In particular, they must be aware of the minimum height

above ground they are to observe. The brief should be

clear and holding no ambiguities.


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3. Map Selection. The routine million map should

be supplemented by a map showing larger details,

map is recommended.

4. Map Study and Preparation. A thorough map

study of intended area to be covered is vital. In addition,

all the hazards to low level flight such as airspace

reservation, airfields, relevant information should be

gathered.

5. Route Selection. Within the limits imposed by

mission briefing, a route should be selected which :-

a. Is favorable as regards terrain and weather.

b. Takes the full advantage of available mapreading features.

c. Avoids airspace reservations, other hazards.

6. Flight Plan

a. Plan as much as possible, to fly at a constant

ground speed for ease in navigational

calculation.

b. Ensure calculating safety altitude.


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c. Obtain flight / route clearance if required.

d. Mark and study diversions.

e. Carryout fuel planning.

f. Workout emergency procedures.


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GPS

1. GPS can operate in part of the World during day ornight in all weather conditions. It takes its data from

satellites revolving around Earth. There are 21 satellites

out of which 18 are active and 3 are reserve. Each

satellite makes 2 revolutions around the earth in one day.

The orbital distance over the Earth is 2000 KM.

2. Basic Components

a. Satellite receiver.

b. Computer.

c. Display and control panel.

d. Power supply.

e. Antennas and cables.

f. Rechargeable batteries with battery charge.

g. Internal antenna.

h. Data transfer cables.

3. Principle of Operation. The receiver gets

information from the satellites and give three

dimensional co-ordinates of the position of the ac. These


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three co-ordinates are longitude, latitude and altitude

(AMSL). This information can only be given if the GPS

is receiving min of four satellites. If GPS is covering

three satellites then it will not give altitude information.

GPS requires minimum of 3 satellites for its operation,

otherwise it gives POOR GPS COVERAGE message.

The satellites are synchronized among themselves and

are transmitting coded pulses. The difference in time of

reception from different satellites will determine the

coordinates. There are two types of codes which are

being sent by the satellites. P-codes. These are only used

by US ARMY are not available to others, C/A codes.These codes are being used by the rest of the

organizations of the World. C/A codes are precise to

hundreds of meters and contain following information :-

a. Position in three dimension.

b. Velocity information.

c. Time information.

4. Features


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a. Feeding of way points by three different

methods.

(1) Lat / Long.

(2) Bearing Distance.

(3) By autostoring.

b. Can set a route comprising of maximum 9

way points.

c. It displays fol navigational information :-

(1) Bearing flown.

(2) Bearing to be flown.

(3) Elapsed time.

(4) ETA.(5) Time to be flown to the destination.

(6) Distance covered.

d. It can autostore any position as a way point.

e. It allows to plan a trip including the fuel

calculations / stops.

f. It can plan vertical navigation.

7/30/2019 Main

main rotor aerodynamics

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